1. Field of the Invention
The present invention relates to an improved binder system for castable ceramics and to an improved method of casting ceramic articles.
In one important aspect, the invention is directed to the casting of refractory ceramic block for lining steel-making furnaces, refractory troughs for molten steel and other similar refractory articles.
As used herein and in the appended claims, the term "ceramic" is intended to mean any material containing refractory metal oxides, carbides, nitrides, etc. which are stable at high temperatures, e.g. above 1000.degree. C. The invention is particularly useful in casting refractory alumina shapes such as furnace lining block, and will be described hereinafter with particular reference to alumina (Al.sub.2 O.sub.3) as the ceramic material.
Heretofore, castable ceramic articles have been made using aqueous binder systems, such as a calcium aluminate/water system, for example, which form a hydrated gel binder. These aqueous binder systems create serious problems when used to cast many refractory articles such as furnace lining block and the like,. The resulting articles are not stable at high temperatures and readily crack and/or spall.
There is therefore a present need in the art for an improved non-aqueous resin binder system for casting ceramic articles. The improved binder system should be capable of producing a castable resin bonded ceramic mix which can be easily poured into complex shapes, if necessary, and cured at relatively low temperature into a rigid body.
2. Description of the Prior Art
It has already been proposed to employ a catalyzed furfuryl alcohol resin binder system for composite articles. For example, in U.S. Pat. No. 4,474,636 issued to B. R. Bogner on Oct. 2, 1984, there is disclosed a glass fiber reinforced composite article employing a binder system composed of hydroxymethylfurfural and an acid catalyst. The catalysts may be toluene sulfonic acid, aniline hydrochloric salts, phenol sulfonic acid, acid chlorides, sulfonic acid salts, hydrochloric acid, phosphoric acid, maleic acid, zinc chloride and the like.
U.S. Pat. No. 4,479,913 issued to Akerberg et al, on Oct. 30, 1984, discloses an organic binder system for carbon bodies which comprises monomeric furfuryl alcohol and a catalyst. The catalyst may be a weak acid such as phthalic anhydride, maleic anhydride or phosphoric acid. Zinc chloride is also mentioned as a suitable catalyst.
In an article published in Industrial and Engineering Chemistry, Vol. 52., No. 1, January, 1960, pages 57 and 58, the authors Goldstein et al. describe their work with monomeric furfuryl alcohol impregnating solutions for porous materials. They concluded that organic acids such as citric and maleic acid and zinc chloride are attractive catalysts for the furfuryl alcohol resin binder used in these impregnating solutions. The solutions where found to exhibit low viscosity and excellent stability at low temperature.
It has been found, however, that furfuryl alcohol catalyzed by reaction with zinc chloride alone does not produce a suitable binder system for casting ceramic articles. Although the catalyzed solution is stable and can be stored indefinitely, zinc chloride does not act as a wetting agent when used in the limited amounts required for the binder system.
It is also known from U.S. Pat. No. 4,474,636, supra, that toluene sulfonic acid is a suitable catalyst for a furfuryl resin binder used in the fabrication of fiber reinforced composites. This catalyst, however, reacts too spontaneously with furfuryl alcohol and consequently, the binder system has too short a working life to be practical.